For granulators and methods of granulation for, for example, urea and urea/ammonium sulfate, in particular, many systems with a combination of a fluidized bed and a spouted bed (hereinafter, which may be referred to as “fluidized bed/spouted bed-type granulator”) have been proposed (see, e.g. JP-B-4-63729 (“JP-B” means examined Japanese patent publication) (Claims (Claim 1), FIGS. 1 to 2), JP-A-10-216499 (“JP-A” means unexamined published Japanese patent application) (Claims (Claims 1 to 3), FIGS. 1 to 3), JP-A-11-137988 (Claims (Claims 1 to 19), FIGS. 1 to 18), JP-A-54-16427 (Claims (Claims 1 to 11), FIG. 1), JP-B-60-13735 (Claims (Claims 1 to 2), FIG. 1), and JP-A-60-97037 (Claims (Claims 1 to 3), FIGS. 1 to 3)), which are preferably practiced currently.
FIG. 1 shows schematically a typical example of such a fluidized bed/spouted bed-type granulator, and the technological characteristics thereof will be described with reference to this figure.
In the figure, for example, seed particles of urea are fed as nuclei to a granulator 1, through a line 41 from a line 40, which is a feed port of the line. In the granulator 1, an aqueous urea solution containing 90% by mass or more, preferably 95% by mass or more, of urea is sprayed as liquid droplets, having a diameter of 150 to 600 μm, to the nuclei at a prescribed spray angle chosen from 30 to 80 degrees, from spray nozzles 6, 7, and 8. Further, an aqueous urea solution (or molten urea) 17, having a concentration of 90% by mass or more, preferably 95% by mass or more, which is fed from a urea synthesis plant or the like (not shown), is set to a temperature of 125 to 145° C., fed from a line 31 to a mixing tank (concentrator) 21, and then fed through a line 36, a pump 22, and a line 37, to the spray nozzles 6, 7, and 8.
Seed particles of urea that are fed through the line 41 grow in its granule size while being subjected to spraying of the aqueous urea solution in the granulator 1. In this growing of the urea granules, by a spouting air flow supplied from the lower inlet of a line 24 through a lower air-supplying pipe 2 and then through air-supplying pipes 3, 4 and 5 branched from the pipe 2, a spouted bed 44 is formed over each of the openings of the air-supplying pipes, and the grown urea granules floating into the space 60 over the spouted beds fall, as the grown product (grown urea granules) 70, to the lower space 11 from the state 10 floating into the upper space. On the other hand, fluidizing air is supplied from the upper inlet of a line 23, jet through a plurality of openings in a bottom floor 9 to the upper space, the bottom floor 9 having the plurality of openings, thereby to form a fluidized bed 12 in which the grown granular urea 70 on the bottom floor 9 is in the fluidized state in the space 11 up to the level 12, and the growing granular urea is fluidized as they occupy the whole space 11 over the spray nozzles 6, 7 and 8.
The bottom floor is generally rectangular in shape, and urea (nuclei) fed to one end of the bottom floor moves continuously in such movement in the fluidized bed over the bottom floor toward the other end of the bottom floor. Thus, the urea (nuclei) moves as it is gradually granulated or enlarged, i.e. grown in granular size (diameter), and finally the granular urea after the completion of granulation is discharged out from an outlet of a line 25.
The proportion of those of a nominal product size among the granular urea discharged from the line 25 of the granulator 1 (hereinbelow, this proportion is referred to as a content of the nominal product size at the granulator outlet.), is generally 75 to 80%, and the granular urea is screened (sifted) through a screen (sieve) 13, to be separated into a standard (on-specification) product and a nonstandard (off-specification) product, with respect to the desired content of the nominal product size in a product. The standard product is passed through a line 26, to be stored as a product 14. On the other hand, to keep the numbers of nuclei in the granulator 1 constant, in view of the stable continuation of the production of the product, the product having a particle diameter greater than the specified particle diameter, and part of the standard product, are passed through a line 27 into a crusher 15, in which they are crushed; the product having a particle diameter smaller than the specified particle diameter is passed through a line 28 and is added to those in a line 29; and the resultant mixture is passed through a line 30 and the line 41 to the inlet of the granulator 1, to be recycled as nuclei for the granulation.